Considerable effort has been made towards the formation of superhydrophobic surfaces because of their potential applications, such as anti-sticking, anti-contamination, and self-cleaning coatings. A superhydrophobic surface is a textured surface that has a water contact angle that is larger than 150° and a low sliding angle, which is the critical angle where a water droplet of a defined mass rolls off the inclined surface. Few efforts have been made towards formation of superoleophobic, or superlyophobic, or even oleophobic surfaces, which display contact angles greater than 90° but less than 150°. Superoleophobic surfaces are also superhydrophobic, and the condition of being superoleophobic allows a superhydrophobic surface to be used for extended periods of time in real-world conditions, such that the superhydrophobic surface maintains its self-cleaning property without oily material accumulation on the surface that can eventually fill the textures, leading to the surface's loss of the superhydrophobic and self-cleaning properties.
Oil repellent surfaces are an engineering challenge because the surface tensions of oily liquids are usually in the range of 20-30 mN/m. The essential criterion, for having a surface with superoleophobicity, is to maintain the oil drops in a Cassie-Baxter (CB) state where vapor pockets are trapped underneath the liquid, which gives a composite surface. The CB state is dependent on the surface's structure and the surface energy of the material. If the structure and surface area are insufficient, the meta-stable energetic state is transformed into Wenzel state.
Superoleophobic surfaces display geometric features having a re-entrant structure, such as mushroom heads, micro-hoodoos, or horizontally aligned cylindrical rods to allow repellency of common hydrocarbon based liquids and oils. The re-entrant structure implies that a line drawn vertically, from the base solid surface through the geometric feature, must proceed through more than one solid interface of that feature. An example of a surface with features defining a re-entrant structure is shown in FIG. 1, where θc>φ is a condition with the interfacial force directed upward, which inhibits the liquid from flowing downward, allowing the possibility of a CB state.
Superoleophobic surfaces require a surface of sufficiently low surface energy relative to the surface energy of oil. The threshold value of the equilibrium contact angle (θtrans) for these surfaces can be calculated by combining the Wenzel and CB equations in the form:cos θtrans=(fs−1)/(Rf−fs)where Rf and fs are the surface roughness factor and fraction of liquid/solid contact area. When Rf>1>fs, cos θtrans<0, the value of the transition contact angle is larger than 90°. This implies that a liquid with surface contact angle smaller than 90° cannot have CB state on that surface. Using Young's equation, the contact angle (θc) is determined by:cos θc=(γSV−γSL)/γLV where γSV, γSL, and γLV are the surface tensions solid/vapor, solid/liquid and liquid/vapor, respectively. For a contact angle of 90°, γSV and γSL are equal; and γSL is approximated as:γSL=γSV+γLV−2(γSVγLV)1/2.The required surface tension of the superoleophobic solid surface is approximately:γSV=γLV/4.′Therefore, to establish a CB state when using a low surface tension liquid, such as octane (21.6 mN/m), the substrates surface tension must be at the range of a few mN/m. Therefore, for oleophobic or superoleophobic surfaces, a fluorocarbon material is required at the surface to sufficiently decrease the surface energy of the structured material.
State of the art superoleophobic surfaces generally lack the ability to be fabricated as a structure in a manner that allows a broad commercial utility, as the techniques used to generate the needed texture are inherently complicated or otherwise disadvantageous with respect to use as a structural component under ambient conditions. For example: Ahuja et al. “Nanonails: A Simple Geometrical Approach to Electrically Tunable Superlyophobic Surfaces” Langmuir 2008, 24, 9-14 and Wu et al. “Design and Microfabrication of High Performance Super-Lyophobic Surfaces” 13th International Conference on Miniaturized Systems for Chemistry and Life Sciences Nov. 1-5, 2009, Jeju, Korea teach plasma or reactive-ion etching of silicon that has a patterned overhang geometry and conformally coated with a fluoropolymer; Leng et al. “Superoleophobic Cotton Textiles” Langmuir 2009, 25, 2456-2460 teaches the formation of silica on cotton cloth by an in situ Stöber Reaction followed by adsorption of silica nanoparticles and treatment with 1H,1H,2H,2H-perfluorodecyltrichlorosilane; Ming et al. “Toward Superlyophobic Surfaces” Contact Angle, Wettability and Adhesion, 2009, 6, 191-205 teaches the formation of raspberry like particles embedding of spherical silica microparticles in a partially cured epoxy resin in a manner that about 40% of the sphere is within the resin and depositing a layer of silica nanoparticles on the microspheres followed by treatment with SiCl4 and 1H,1H,2H,2H-perfluorodecyltrichlorosilane; Tuteja et al. “Designing Superoleophobic Surfaces” Science 2007, 318, 1618-1622 teaches microspinning microfibers onto a surface with a frilly 1H,1H,2H,2H-heptadecafluorodecyl substituted polyhedral oligomeric silsesquioxane (RfPOSS)/polymethylmethacrylate (PMMA), which phase separation of the RfPOSS occurs during microspinning to form a superoleophobic surface when the RfPOSS portion exceeds about 10%; and Xie et al. “Facile Creation of A Super-Amphiphobic Coating Surface with Bionic Microstructure” Adv. Mater. 2004, 16, 302-305 teaches the deposition of a 5% PMMA/perfluorocarbon end-capped polyurethane FPU solution in dimethylformamide (DMF) on a glass substrate as a rough micro-nano binary structure of micro-papilla of PMMA covered by a rough conformal coating of the FPU due to microphase separation during the slow ambient air evaporation of the DMF from the solution. Hence, an article having an oleophobic superhydrophobic surface, which can be prepared by an easily performed method, is needed for the development of useful articles requiring superhydrophobic surfaces that are sufficiently robust for use in the ambient environment.